Method of drilling



Jan. 25, 1966 c. J. BLOM 3,231,030

METHOD OF DRILLING Filed Sept. 28, 1961 FRANGIBLE DRILLING OBJECTS MUD 50\ 41 4o 37 as PUMP INVENTOR CHRIST/AN JAMES BLOM ATTORNEY United States Patent 3,231,030 METHOD OF DRILLING Christian James Blom, Bakersfield, Califi, assignor to Chevron Research Company, a corporation of Dela- Ware Filed Sept. 28, 1961, S81. No. 141,409 4 Claims. (Cl. 175-65) This invention relates to improved methods and apparatus for drilling holes in the earths crust and more specifically, this invention relates to methods and apparatus for utilizing the potential energy of a column of drilling mud contained in a borehole during the drilling thereof to provide a plurality of high energy shock waves in the vicinity of the drilling face of said borehole to thereby promote more rapid rock breakage at said face resulting in an improved drilling rate.

In nearly all conventional earth drilling operations a hole is advanced into the earth by force exerted at the drilling face by a drill bit. Generally the force is supplied to the drill bit by means of energy transmitted from the surface through a drill stem or some similar means. The weight applied to the drill bit by and through the drill stem increases the force with which the bit acts on the drilling face. In relatively hard, nonductile rocks the hole is advanced by a combination of mechanical abrasion of the rock and by tension and/or shear fracturing in the rock both caused by the force of the drill bit acting on the drilling face.

Further, in conventional wet drilling operations a drilling fluid, commonly known as drilling mud, is utilized to carry the chips and rock dust from the drilling face up the borehole to the surface. Drilling mud also serves to lubricate the drill bit, to prevent formation fluid from entering the borehole, to prevent relatively plastic formations from pinching into the drill string and to prevent shaley formations from scaling into the hole. Drilling mud is conventionally circulated down the interior of the drill stem to the drilling face and thence up the annulus between the hole wall and the drill stem to the surface. In deep holes such as are common in the drilling art the drilling mud is continuously circulated from the surface to the drilling face and back to the surface. Therefore at any given time there is a column of drilling mud in the hole equal in height to the depth of the hole. This column of drilling mud often exceeds 2000 feet in depth. Since drilling mud is commonly a very dense fluid a tremendous amount of potential energy is therefore present in the vicinity of the drilling face near the lower end of said drilling mud column due to the weight of the column of mud in the hole. Heretofore this potential energy has not been utilized during drilling operations. The drilling mud has serve-d only the above recited functions of lubricating the drill bit, removing cuttings from the hole and retaining hole competence. No effective utilization has been made of the tremendous energy of the mud column potentially available at the drilling face.

One means of utilizing the large potential energy existing in a column of fluid is known as implosion. An implosion is caused by suddenly opening a relatively low pressure zone to the much higher pressurized surrounding. For example, the drilling mud in the borehole near the drilling face is at an extremely high pressure due to the weight of the column of mud in the hole. If a significant zone having a relatively low internal pressure compared to the pressure existing in the mud is established near the drilling face and then if said zone is suddenly opened to the high pressure drilling mud, a sharp implosion shock is caused by the inrush of the drilling mud to fill the zone previously at said relatively low pressure. The shock resulting from this implosion is very substan- 3,231,03ii Patented Jan. 25, 1966 tial. The present invention provides methods and apparatus for causing implosions in the vicinity of a drilling face and directing the resulting shock waves at the drilling face to thereby utilize the potential energy of the drilling ifnud column to aid in breaking the rock at the drilling ace.

Therefore, it is a principal object of the present inven tion to provide methods and apparatus for establishing a plurality of relatively low pressure zones adjacent a drilling face during the drilling of a borehole and to suddenly open said zones to the surrounding high pressure environment therein to thereby effectively utilize the potential energy of the column of drilling mud in said borehole to create high energy shock waves in the vicinity of said drilling face of said borehole during the drilling thereof resulting in larger stresses in the rock near said drilling face and improved rock breakage at said face thus increasing the drilling rate in said borehole.

In one aspect of the present invention provides for the improvement in drilling into an earth formation wherein liquid is circulated in a borehole comprising adding pressure-resistant frangible objects to said circulating liquid in the course of drilling said borehole, said objects having a relatively low internal pressure compared to the downhole pressure of said circulating liquid, flowing said circulating liquid containing said frangible objects to the vicinity of said drilling face of said borehole to establish therein a plurality of relatively low pressure zones and breaking said frangible objects under said drill bit in the vicinity of said drilling face to cause said circulating liquid to suddenly occupy the zone theretofore taken up by said frangible objects whereby the resulting implosions cause stresses in the rock at said drilling face to thereby allow said drill bit to more readily break said formation.

Further objects and advantages of the present invention will become apparent from the following detailed description read in light of the accompanying drawing of which FIG. 1 is a vertical View partly in section of an earth formation penetrated by a borehole.

FIG. 2 is a vertical view partially in section of one cone of a drill bit, said bit useful in the present invention.

FIG. 3 is a sectional view of a drill bit roller, a frangible object, and the drilling face of a borehole.

Referring now specifically to FIG. 1, an earth formation 26 is shown penetrated by a borehole 22. Drill bit 24 is seen in operable position at drilling face 26. Drilling is accomplished by rotating drill bit 24 against the drilling face 26 to grind and chip away the rock therepresent. Rotative force is transmitted to hit 24 by drill stem 28 which connects drill bit 24 to rotary table 29 located at the earths surface. Rotary table 29 is driven by motor 27 both of which are supported by a conventional drilling rig such as drilling rig 30. The rotary table 29 is rotated and the rotational motion is transmitted to bit 24 by means of drill stem 28. The weight applied to bit 24 by means of drill stem 28 creates locally highly stressed regions at the points of contact of bit 24 with the drilling face thereby aiding bit 24 in grinding and chipping the rock at drilling face 26.

Drilling mud is circulated down through the interior of drill stem 28 to drilling face 26 and is returned to the surface up the annulus between drill stem 28 and borehole 22 to the surface from where it may be recirculated. The primary purpose of circulating drilling mud in the borehole has heretofore been to remove cuttings from drilling face 26, to lubricate drill bit 24 and to provide for hole competence during drilling. The method of the present invention provides for utilizing the potential energy of the drilling mud in the form of implosion-created shock waves to promote more rapid breakage of the rock at drilling face 26. The present invention provides for establishing a plurality of zones of relatively low pressure compared to the pressure of the drilling fluid in the vicinity of drilling face 26. The low pressure zones are preferably established between drill bit 24- and drilling face 26. The low pressure zones established between the drill bit 24 and drilling face 26 are suddenly opened to the downhole drilling mud which is at much higher pressure. The resulting inrush of drilling mud into the zone theretofore at relatively low pressure causes shock waves to be set up in the drilling mud resulting in stresses in the rock at the drilling face. These shock waves aid drill bit 24 in breaking the rock at drilling face 26.

A preferred form of apparatus for practicing the method of the invention comprises pump 32 which is in fluid communication with drill stem 28 by means of conduit 33. A supply of drilling mud 34 is provided for pump 32 through valve 40 on conduit 35. A source of pressureresistant frangible objects 36 which will be fully described hereinafter is in communication with pump 32 through valve 41 on conduit 37. Drilling mud is circulated to drill bit 24 at the drilling face 26 where pulverized earth and chips which are broken from drilling face 26 as bit 24 proceeds into the earth are picked up in the mud and carried from the hole.

At selected intervals in the course of drilling borehole 22 pressure-resistant, frangible objects having a relatively low interior pressure are added to the drilling mud at pump 32 from source 36 for circulation with said mud through conduit 33, drill stem 28 to the vicinity of drilling face 26. The hollow frangible objects are preferably broken between drill bit 24 and the drilling face 26 to thereby open communication between the relatively low pressure zone theretofore in the interior of a pressureresistant, frangible object and the high pressure drilling mud. The resulting sudden inrush of said mud causes shock Waves to be initiated in the mud and in rock near drilling face 26. The increased tension and compression stress on the rock aids drill bit 24 in grinding and chipping said rock. A preferred interval in the course of drilling a borehole at which it is desirable to add pressure-resistant frangible objects to the circulating drilling mud is when the drill bit encounters exceedingly hard and durable rock.

The stresses established in the rock by the implosion of the frangible objects aid the bit in breaking the durable rock. The shock waves cause the hard durable rock to chip more readily under a drill bit than is found when only conventional methods are used. The frangible objects are desirably added to the circulating drilling mud when the drill bit encounters subsurface formations composed of durable rock which rock is particularly resistant to grinding and chipping by conventional methods of liquid circulated drilling. Persons skilled in the drilling art may become aware of the presence of the drill bit at these rock formations by indications transmitted up the drill stem.

A further interval in the course of drilling a borehole at which it is desirable to add frangible objects to the drilling mud occurs when the rollers of a conventional rotary bit are jammed with debris from the bottom. This most frequently occurs in clay-bearing formations. An addition of frangible objects to the circulating mud and the implosions resulting by breakage of said objects in the presence of drilling mud at the bit will aid in cleaning the rollers in the hole as drilling proceeds. The ability to clean the bit in the hole obviates the necessity of pulling the entire drill string to clean the bit and thereby saves large expenditures of time and money.

As indicated above one means for establishing zones of relatively low pressure adjacent the drilling face is adding pressure-resistant objects, said objects having a relatively low internal pressure, to the drilling mud which is circulated to the drilling face in the course of drilling a borehole. In order that the low internal pressure zone in the pressure-resistant objects be selectively communicable with the high pressure drilling mud at the drilling face the objects are desirably frangible. Communication between the high pressure fluid and the relatively low pressure zone is established by breaking the object. The objects are preselected to withstand the pressure of the mud at the bottom of the hole and to be breakable when caught between the drilling face and drill bit.

A preferred material for construction of the frangible objects is glass. Glass is easily molded to a desired shape and thickness. The frangible objects are desirably spherical in shape. The interior of the frangible objects is maintained at a relatively low pressure compared to' the bottom hole pressure of the drilling mud. The interior of a frangible object is preferably filled with a gaseous medium usually air. The pressure of the gaseous medium is adjusted to a predetermined low pressure which will insure that the internal pressure of the object will be greatly less than the pressure of the drilling mud at the drilling face. Usually the internal pressure of the objects should approximate atmospheric pressure. The interior need not be filled with a gaseous medium. Under some conditions it is desirable to provide the frangible object with some additional internal strength. It is important however that the objects interior be maintained at a relatively low pressure compared to the bottom hole pressure of the drilling mud.

The optimum size of the frangible objects will range depending on a number of factors including hole diameter and depth, type of bit used in drilling the hole, type of rock expected to be encountered and other factors. Generally, good results can be obtained using frangible objects shaped as spheres having a diameter as smali as A of an inch. Larger sized objects, however, are s0metimes preferred since a larger zone at a relatively low internal pressure may be provided in the borehole thereby resulting in more intense shock waves as the larger objects are imploded. A size range of from inch outside diameter to one inch outside diameter is preferred for use in most applications of the invention with conventional rotary drilling methods. The wall thickness which controls the size of the zone of relatively low internal pressure for a given diameter is preselected depending on borehole conditions, for example, the pressure exerted by the column of drilling mud. Larger objects ranging in size up to a maximum size object that can'be readily positioned between the drilling face and the drill bit while the drilling operation is proceeding are sometimes desirable. The maximum size objects which are useful in the method of the present invention therefore are limited in size to objects which can conveniently be circulated to the drilling face.

An example not intended to limit the present invention but by way of illustration of the pressure exerted at implosion of an object of the type provided by the present invention follows. In selecting objects for a specific drilling operation the pressure of the drilling mud in the vicinity of the drilling face must be determined. One skilled in the art of drilling, knowing the density of the mud and the depth of the borehole, can easily calculate the bottom hole mud pressure. For purpose of this example the drilling mud pressure is taken as 7000 pounds per square inch (p.s.i.). Further, for purpose of the example it is found desirable to utilize pressure-resistant spherical objects 0.25 inch in diameter and .01 inch wall thickness.

The stress in the hollow spheres under external pressure is at a maximum at the inner surface and has a value of:

M =Maximum stress (tangential to inner surface) R =Inner radius of hollow ball b=Outer radius of hollow ball P =External pressure P =Pressure of surrounding fluid =Density of fluid R =Initial inner radius for this example let =.0464 lb./in. (80 lbs. per ft.

=48,500 p.s.i.

where T=2.6- seconds During collapse a very high pressure will be developed which approximates:

where P=The high pressure developed. R=The instantaneous inner radius of the bubble.

This spherical surface of high pressure occurs at a radius:

r: l .5 9R

where r=the radius of the high-pressure surface.

When R=R in the case of this example (i.e., when the sphere has collapsed so that the radius is of the original 0.115 inch radius):

The compression of fluid during this intense implosion will result in a damped series of implosion-explosions. The first implosion in the cycle will have the greatest in tensity and if the implosion occurs near a reflecting parabolic-shaped surface it will create a jet focused by the parabolic surface which will act on the drilling face in a manner similar to a shaped charge explosive. As is evident from the above example, large forces may be exerted on the drilling face by means of imploding relatively small objects between the drilling face and a drill bit. The calculations and constants referred to above were obtained from an article by Lord Rayleigh in the Philosophical Magazine, 1917, volume 34 at page 94, and from Theory of Elasticity by Timoshenko and Goodier, second edition, 1950, pp. 356-359.

The implosion can be intensified by reflection from a directional reflecting surface. However, even without a special reflecting surface the implosion acts to greatly improve drilling, Therefore, it is in keeping with the spirit of the present invention to utilize the pressureresistant objects either with a convenient drill bit or with a drill bit provided with special reflecting surfaces. Said reflecting surfaces may take many forms, however, it has been found that parabolic cavities formed in the drilling face contacting surfaces of a drill bit are especially desirable to increase the intensity of the shock waves produced by an implosion therein.

P =s,s00,000 p.s.i.

FIG. 2 is a vertical view partially in section of one cone of a drill bit of a type useful in practicing the present invention. For convenience FIG. 2 shows only one cone of a mono or multi-cone bit. The bit, as is known to those skilled in the art, might be made up of one or more such cones. As shown in FIG. 2, is the collar connecting the drill bit with the lower end of a drill stem. Bit body 52 is adapted to fit into collar 50. Rotatively mounted on bit body 52 is bit roller 53. Roller 53 may take the shape of a truncated cone. On the earth contacting surface of roller 53 are special reflecting surfaces 54 and protrusions 55. As indicated heretofore the implosion resulting from breaking one of the objects as taught in this invention can be directed at the drilling face with greater force than is otherwise possible if a reflecting surface is provided in the bit roller. The reflecting surfaces may take any appropriate shape. Reflecting surfaces in the shape of parabolic cavities have been found to be particularly desirable. The parabolic reflecting surfaces shown as 54 in FIG. 2 are located on the earth contacting surface of roller 53. The reflecting surfaces 54- are located with respect to protrusions 55 so as to'be directly in front of each protrusion as it contacts the drilling face as bit roller 53 rolls over the face. The drill bit is adapted to be connected to a drill stem and to be rotated against the drilling face. The rock at the drilling face is chipped and pulverized in this manner to thereby advance the hole into the earth. In practice the method of the invention is not limited to use of a drill bit having specially constructed reflecting surfaces such as parabolic cavities 54. The implosions may be generated by conventional bits used in combination with the pressure-resistant objects. The resulting stress in the rock at the drilling face, while not as great as the stress created when the shock waves are directed, is still helpful in improving the drilling rate.

Referring specifically to FIG. 3 a sectional view of bit roller 53, pressure-resistant object 57 and drilling face 26 is shown. The preferred positions of reflecting surfaces 54 and protrusions 55 are illustrated with respect to the direction of roller rotation. The reflecting surface is desirably a parabolic cavity. The focus of the parabolic cavity is directed at the rocks surface in order that the major force of the implosion is directed so as to concentrate the stress field in the rock near protrusion 55. It is believed that in this manner the major effect may be obtained from the implosion resulting when the object 57 is crushed in cavity 54. A desirable size relationship between the parabolic cavity and the frangible object is also evident in FIG. 3. The object is sized to fit partially into the cavity. Enough of the object should protrude from the cavity to insure that the object breaks as it contacts the drilling face.

As discussed above the pressure-resistant frangible objects are added to the drilling mud at selected intervals during the course of drilling a borehole. The objects are a desirable addition to the drilling mud when tough durable rock is encountered. The rate at which the frangible objects should be added to the circulating drilling mud depends on a number of variables including hole diameter, bit revolutions per minute, particular formation rock and other variables. :It is preferred when using a bit having reflecting cavities as described above to add the frangible objects to the circulating mud in sufiicient number to have at least one frangible object in the mud per each cavity contacting the drilling face. The invention is not limited to a particular rate of addition of the objects to the drilling mud. Obviously this will vary depending on the type of formation being drilled. A relationship between a desirable rate of addition of frangible objects to the circulating mud and the particular formation being drilled can be established experimentally by one skilled in the drilling art.

The invention has been described and illustrated using a few specific examples of desirable apparatus with which to perform the invention and a few desirable intervals in the course of drilling at which to perform the method. The method of the invention is not to be limited to those specific instances but rather it is to be accorded a scope commensurate with the appended claim.

The invention having been described and illustrated, I claim:

1. Apparatus for drilling a borehole comprising a drill bit in operational relationship with the drilling face of a borehole, means for operating said bit, a drilling fluid, means circulating the drilling fluid in said borehole between the earths surface and the drilling face of said borehole, a plurality of pressure-resistant hollow frangible objects entrained in said drilling fluid for circulation with said drilling fluid to said drilling face of said borehole, and a plurality of parabolic-shaped cavities in said drill bit for receiving said hollow frangible objects.

2. Apparatus for drilling a well comprising a drill bit in operational relationship with the drilling face of a well, means for operating said bit, a drilling fluid, means for circulating the drilling fluid in said well between the earths surface and the drilling face of said well, a plurality of pressure-resistant hollow frangible objects entrained in said drilling fluid for circulation with said fluid to said drilling face of said well and directional reflecting surface means formed in said drill bit for receiving said hollow frangible objects therein and for directing at the drilling face the force of the implosions resulting when said hollow frangible objects are broken.

3. A method of drilling a well comprising forcing a drill bit against a drilling face of a bore hole, circulating a drilling mud in said borehole to said drilling face, entraining hollow frangible objects in said circulating mud, flowing said mud containing said frangible objects to said drilling face, breaking said objects between said drillface and said drill bit, directing the shock waves resulting from the imp-losions caused by breaking said frangible objects at the drilling face and intensifying the effect of said shock waves by reflecting said shock waves from directional reflecting surfaces formed in said bit.

4. A method of drilling a borehole into the earth, which method includes the step of circulating a drilling mud from the surfaceof the earth to the drilling face of said borehole comprising adding pressure-resistant frangible objects to the circulating drilling mud, said objects having a relatively low internal pressure compared with the pressure of the drilling mud near said drilling face, flowing the drilling mud containing said frangible objects to a position in said borehole nearsaid drilling face, breaking said frangible objects with said drill bit to cause an implosion of drilling mud between said drill bit and said drilling face 'Whereby said drilling face is more easily broken by said drill bit as a result of the shock waves of said implosion and intensifying the effect of said shock waves on said drilling face by reflecting said shock waves from a directional reflecting surface formed in said drill bit.

References Cited by the Examiner UNITED STATES PATENTS 2,187,432 1/1940 Powers -21 2,233,260 2/1941 Hawthorne 175-54 2,361,558 10/1944 Mason 166-43 2,621,351 12/1952 Piety 166-65 X 2,634,098 4/1953 Armentrout 175-72 2,642,268 6/1953 Armentrout 175-72 2,676,892 4/1954 McLaughlin 65-21 X 2,754,910 7/1956 Derrick et al 166-42.1 2,800,449 7/1957 Browning 252- 2,828,258 3/1958 Thompson 252-85 2,836,555 5/1958 Armentrout 175-72 X 3,083,778 4/1963 Friedman et a1 175-2 3,174,561 3/1965 Sterrett 175-65 OTHER REFERENCES Setser, Donald D.: Implosion Technique Improves Fracturing Performance, in World Oil, March 1960, pages -106.

CHARLES E. OCONNELL, Primary Examiner.

BENJAMIN BENDETT, Examiner. 

3. A METHOD OF DRILLING A WELL COMPRISING FORCING A DRILL BIT AGAINST A DRILLING FACE OF A BORE HOLE, CIRCULATING A DRILLING MUD IN SAID BOREHOLE TO SAID DRILLING FACE, ENTRAINING HOLLOW FRANGIBLE OBJECTS IN SAID CIRCULATING MUD, FLOWING SAID MUD CONTAINING SAID FRANGIBLE OBJECTS TO SAID DRILLING FACE, BREAKING SAID OBJECTS BETWEEN SAID DRILLFACE AND SAID DRILL BIT, DIRECTING THE SHOCK WAVES RESULTING FROM THE IMPLOSIONS CAUSED BY BREAKING SAID FRANGIBLE OBJECTS AT THE DRILLING FACE AND INTENSIFYING THE EFFECT OF SAID SHOCK WAVES BY REFLECTING SAID SHOCK WAVES FROM DIRECTIONAL REFLECTING SURFACES FORMED IN SAID BIT. 